1. Technical Field
The present invention relates to a hydrostatic gas bearing for use in a vacuum, a hydrostatic gas bearing device incorporating the hydrostatic gas bearing, and a gas recovering method for the hydrostatic gas bearing device.
2. Background Art
Conventionally, in a hydrostatic gas bearing for use in a vacuum, as a shape accuracy required for a moving body and a fixed body surface opposed to the moving body, only flatness to an extent of not affecting the size of a floating gap and a labyrinth seal gap has been required, and pores in the surface have especially not been considered. Also, in selecting the material of a guide shaft of the fixed body and the material of the moving body, only a property that the quantity of gas being discharged when the material is continuously exposed to a vacuum environment for a fixed period of time or more is small has been considered not to exert an influence on the reached degree of vacuum of mounted equipment.
Also, a general hydrostatic gas bearing is configured so that a high-pressure gas is released from an air pad provided on the moving body into a gap between the moving body and the fixed body surface, and the moving body and the fixed body are made in a non-contact state by the hydrostatic pressure of released gas, by which the moving body is moved with frictional resistance being scarcely encountered.
In the hydrostatic gas bearing as described above, in order to maintain the vacuum environment, it is desirable that an atmospheric pressure groove be provided around the air pad, and a mechanism for keeping the gas pressure in the atmospheric pressure groove at the atmospheric pressure provided.
The reason for this is that by providing the atmospheric pressure groove around the air pad, even in the design of hydrostatic gas bearing for vacuum, the load capacity, rigidity, and the like of the air pad can be designed by the same design technique as that for the ordinary hydrostatic gas bearing for use in an atmospheric environment.
Also, when the atmospheric pressure groove is not provided, all gas emitted from the air pad flows into an exhaust pump through a pressure reducing groove, so that the load of the exhaust pump increases undesirably. To prevent this phenomenon, the provision of the atmospheric pressure groove is also effective.
Therefore, as a mechanism for keeping the pressure in the atmospheric pressure groove at the atmospheric pressure, conventionally, an atmospheric pressure pipe has been connected to the atmospheric pressure groove provided around the air pad, and the other end of the atmospheric pressure pipe has simply been opened to the atmosphere on the outside of a vacuum chamber.
For the hydrostatic gas bearing manufactured by using the prior art, even when the quantity of gas discharged from a bearing device is lower than the specified discharge quantity in a stationary state, the quantity of gas discharged from the bearing device increases suddenly at the same time that the moving body starts to move, so that the reached pressure in the vacuum chamber is sometimes deteriorated remarkably. The reason for this is as described below. For example, in the hydrostatic gas bearing provided with the air pad on the moving body side, a portion covered by the moving body of the fixed body surface is exposed to a high-pressure floatation gas in a stationary state, and thus large quantities of gas molecules adhere to that portion. Subsequently, when the moving body moves and the surface thereof is exposed to the vacuum environment, large quantities of gas molecules adhering to the surface of guide shaft are discharged to the outside, which is resultantly detected as a sudden increase in the quantity of gas being discharged.
When the above-mentioned pressure increase occurs in a hydrostatic gas bearing mounted, for example, in an EB exposure system, a sudden increase in pressure degrades the accuracy of exposure pattern, and sometimes damages the electron beam source.
A hydrostatic gas bearing constructed so that the air pad is provided on the fixed body side also presents the same problem at the time of movement because gas molecules adhere to the surface of the moving body.
The inventor started research and development of a hydrostatic gas bearing capable of being used in an environment of higher vacuum (about 10−5 Pa). As the result, it was verified that there occurs, on rare occasions, a phenomenon that the gas pressure in the vacuum chamber increases due to the movement of the moving body even from a cause other than the gas supplied to the air pad.
Thus, in order to determine the cause and to provide a higher-performance hydrostatic gas bearing, the inventor further conducted research and development earnestly.
For example, in the case of a semiconductor exposure system, the vacuum chamber in which a hydrostatic gas bearing is mounted is usually placed in a clean room whose temperature and humidity are controlled.
In the hydrostatic gas bearing in which the atmospheric pressure groove in the bearing is simply opened to the atmosphere on the outside of vacuum chamber via the atmospheric pressure pipe, gas usually flows out from the bearing side to the atmosphere side. However, it was found that gas may sometimes flow in inversely
from the atmosphere side to the bearing side depending on the pressure balance of the atmospheric pressure groove fluctuated by the bearing gap etc.
When gas flows inversely from the atmosphere side to the bearing side as described above, the atmosphere having a specific humidity flows in to the bearing side via the atmospheric pressure groove, and thus large amounts of water molecules undesirably adsorb on the surface of the guide shaft. If the moving body of the bearing starts to move in this state and the guide shaft surface on which water molecules have adsorbed is exposed to a vacuum, the adsorbing water molecules are released into the vacuum environment, which results in an increase in gas pressure in the vacuum chamber.
Even if the quantity of adsorbing water molecules is minute, when the pressure in the vacuum chamber is a high vacuum of about 10−5 Pa, a remarkable increase in pressure is observed. If such an increase in pressure occurs in the bearing, for example, in an electron beam exposure system, the above-mentioned phenomenon may cause a failure of light source and degradation of exposure accuracy.
In the hydrostatic gas bearing for use in an environment of an especially high vacuum, it is necessary to carry out control so that the causes for the increase in gas pressure in the vacuum chamber are obviated.